This is how you get into trouble: Defining Art.  All we assert is that these are great databases that have some information on artists or image libraries.  You'll know the rest when you see it.

New York's Metropolitan Museum of Modern Art 

http://www.metmuseum.org/toah/splash.htm

The MoMA has been generous enough to share this with us on-line.  It has a well organized time-line of art movements and artists.

Art Encyclopedia 

http://www.artcyclopedia.com

A comprehensive collection of art that is searchable by artist, title of of the work and even the Museum where the piece currently resides. 

New York Public Library Visual Archive 

http://digital.nypl.org/mmpco/

The New York Public Library has assembled this image databse.  It is organized according to general category as well as key word.  This easily turns into a huge time waster; which is not necessarily a bad thing.